Insulin and leptin are key signals in the body’s regulation of weight by influencing the brain. Insulin acts on the hypothalamic melanocortin system, particularly on neurons with insulin receptors that also express POMC. When insulin is administered, there is an increase in POMC mRNA expression. Blocking melanocortin receptors can prevent the weight-reducing effects of insulin.
The control of body weight involves peripheral signals and central pathways that manage food intake and energy expenditure. Insulin release is linked to adipose tissue mass, with plasma levels fluctuating according to energy balance. Insulin can cross the blood-brain barrier to affect CNS systems that govern weight regulation.
Central insulin administration can lead to lower food consumption and weight, while interference with insulin receptors or signaling molecules can result in weight gain and obesity. Insulin serves as a form of negative feedback to the CNS based on energy reserves.
Similarly, leptin reduces food intake and weight by acting centrally. The hypothalamic melanocortin system is believed to be involved in mediating the impacts of both leptin and insulin on food consumption and weight regulation.
Experimental Procedures
Subjects
Male Long-Evans rats were utilized in the research, with food and water available ad libitum.
Experiment 1: Dual-Labeling Immunohistochemistry
Investigation of insulin receptors and POMC expression in hypothalamic arcuate neurons through immunohistochemistry.
Experiment 2: POMC Expression
Study on the impact of central insulin administration on POMC mRNA expression using i3vt injections and RNA quantification.
Analysis of POMC expression was conducted through Taqman real-time PCR. POMC was amplified in triplicate RNA samples from each rat using forward and reverse primers with a Taqman probe. GAPDH served as a reference gene. Reactions were performed with the Taqman EZ RT-PCR core reagent kit. Standard curves were generated from pooled RNA of each treatment group. Amplification sequence and fluorescence detection were carried out using the ABI PRISM 7700 Sequence Detection System. Baseline and threshold fluorescence were determined by the system. Relative expression was calculated based on the cycle number surpassing the threshold. The percentage change in POMC expression relative to the reference group was analyzed using one-way ANOVA.
In Experiment 3, a subthreshold dose of a melanocortin antagonist was employed to block the ability of central insulin to reduce food intake. Surgeries for intracerebroventricular cannulations were performed, and SHU-9119 was administered prior to food intake measurements.
The co-localization of POMC and the insulin receptor was assessed through dual-labeling immunohistochemistry. Immunoreactivity for the insulin receptor-β was observed in brain regions known to have mRNA or binding sites, such as the hippocampus and arcuate nucleus. Arcuate neurons expressing POMC also exhibited immunoreactivity for the insulin receptor-β.
Hypothalamic POMC expression was analyzed after feeding, fasting, and fasting with insulin infusions. Insulin administration led to increased POMC expression compared to fasting. SHU-9119 did not impact food intake on its own but reversed the reduction caused by insulin.
The effects of insulin and SHU-9119 on food intake were measured, with insulin significantly reducing intake while SHU-9119 blocked this effect when administered before insulin.
The discussion highlighted the role of insulin in regulating intake and weight, emphasizing similarities with leptin’s actions in the CNS. Specific neuronal populations in the ARC of the hypothalamus, particularly POMC-expressing neurons, were identified as playing a part in mediating the catabolic effects of leptin. This suggests a collaborative effort between insulin and leptin in regulating caloric intake and expenditure.
Leptin receptors present in POMC neurons in the ARC stimulate gene expression and boost electrical activity. Blocking melanocortin receptors inhibits the effect of leptin on neuronal activation and food intake. Insulin interacts with the central melanocortin system to manage energy balance, with insulin receptors located on POMC neurons. Some insulin receptor-containing cells do not contain POMC, implying the involvement of other ARC systems like neuropeptide Y and AgRP.
Insulin possibly influences the melanocortin system by regulating POMC expression. It can counteract the decrease in POMC expression during fasting, stimulate POMC gene expression, and potentially reduce food intake and weight by modulating the central melanocortin system.
Insulin’s central catabolic effects are mediated through melanocortin receptors, similar to the action of leptin. These studies demonstrate that the CNS melanocortin system is a critical downstream target for insulin’s impact on food intake and weight regulation.